Detectors which are capable of detecting single photons so-called single photon detectors are an important component of any system which operates using the principles of quantum cryptography. Such systems rely upon the transmission of data bits as single particles, in this case, photons, which are indivisible. The data may be encoded using polarisation of the electric field vector of the photons, the phase of the photons etc.
However, as well as detecting a single photon, there is also a need to produce detectors which are capable of resolving the number of photons in a pulse of radiation. Such detectors are useful for characterising non-classical light sources such as single photon generators to determine if they are genuine and reliable single photon sources. Another potential application is for determining if one or more photons are received per pulse in a quantum communication system. If two or more photons are present in a pulse then the pulse may be subject to a pulse splitting attack where just one photon from the pulse is read by a eavesdropper. This seriously degrades the security of the system. Thus, it is desirable to develop a detector which allows the number of photons in a pulse to be accurately determined.
Single photon detection is also useful as a low level light detection means for spectroscopy, medical imaging or astronomy. Both in medical and astronomical applications the high energy photons (X-ray etc) or high energy particles are converted in scintillators into many (10-100) low energy photons. These low energy photons are then detected by avalanche photodiodes or photomultiplier tubes. As the low energy photons that are produced are scattered in space there is a need for large area detectors which are very sensitive. Also arrays of such detectors allow the spatial distribution of low energy photons to be obtained in order to gain information about the original photon. These applications would also benefit from the ability to count the number of photons incident on a single photon detector.
Previous attempts to develop a single photons detector which is capable of determining the number of photons detected from a pulse include single photon avalanche photodiodes (APDs) operated in the co-called “geiger mode”. These detectors are binary (“click counting”) detectors, several schemes have been proposed involving either multiple devices or time multiplexing to allow photon number counting. However such schemes cannot resolve two photons incident on the same detector at the same time.